/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Preconditions;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.concurrent.Executor;

/**
 * A {@link ListenableFuture} which forwards all its method calls to another future. Subclasses
 * should override one or more methods to modify the behavior of the backing future as desired per
 * the <a href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p>
 * Most subclasses can just use {@link SimpleForwardingListenableFuture}.
 *
 * @author Shardul Deo
 * @since 4.0
 */
@CanIgnoreReturnValue // TODO(cpovirk): Consider being more strict.
@GwtCompatible
public abstract class ForwardingListenableFuture<V> extends ForwardingFuture<V> implements ListenableFuture<V> {

    /** Constructor for use by subclasses. */
    protected ForwardingListenableFuture() {}

    @Override
    protected abstract ListenableFuture<? extends V> delegate();

    @Override
    public void addListener(Runnable listener, Executor exec) {
        delegate().addListener(listener, exec);
    }

    // TODO(cpovirk): Use standard Javadoc form for SimpleForwarding* class and constructor
    /**
     * A simplified version of {@link ForwardingListenableFuture} where subclasses can pass in an
     * already constructed {@link ListenableFuture} as the delegate.
     *
     * @since 9.0
     */
    public abstract static class SimpleForwardingListenableFuture<V> extends ForwardingListenableFuture<V> {
        private final ListenableFuture<V> delegate;

        protected SimpleForwardingListenableFuture(ListenableFuture<V> delegate) {
            this.delegate = Preconditions.checkNotNull(delegate);
        }

        @Override
        protected final ListenableFuture<V> delegate() {
            return delegate;
        }
    }
}
